BALTIMORE – A bright, sunny afternoon at Pimlico that dawned with the promise of greatness suddenly, tragically, turned into one of the darkest days in the 300-year history of thoroughbred racing when Kentucky Derby winner Barbaro, seemingly on course to become the first Triple Crown winner in 28 years, fractured his right hind leg soon after the start of yesterday’s 131st Preakness.

The race was won by Bernardini, making just his fourth lifetime start. He came from just off the pace with a sweeping move on the far turn under Javier Castellano to score by 51/4 lengths over beaten Derby favorite Sweetnorthernsaint, with Hemingway’s Key another six lengths back in third and Brother Derek fourth.

But the victory, before a record crowd of 118,402, was overshadowed by Barbaro’s injury, termed “life-threatening” by attending veterinarian Dr. Larry Bramlage.

Things went awry even before the race when Barbaro, previously undefeated in six starts and the heavy favorite at 1-2 in a field of nine, broke through the gate. He was reloaded, then was away cleanly and appeared to be running smoothly for the first eighth of a mile when he was abruptly eased by jockey Edgar Prado.

But Barbaro continued running for several strides, compounding the injury. As trainer Michael Matz rushed to the track, the strapping son of Dynaformer stood on three legs, lifting the injured limb in the air.

“When he went to the gate, he was feeling super,” Prado said. “He tried to buck me off a couple of times, he was feeling that good. He just touched the front of the doors of the gate and went right through it.

“During the race, he took a bad step. I can’t really tell you what happened. I heard a noise about 100 yards into the race and pulled him right up.”

A temporary cast was placed on the leg, and Barbaro was loaded into the horse ambulance. After X-rays were taken, the ambulance raced up the highway to the University of Pennsylvania’s New Bolton Center in Kennett Square, Pa., one of the nation’s top equine clinics, where hopefully surgery can be performed to save his life.

“He’s has fractures above and below his ankle,” Bramlage said. “The more steps he takes on an unstable limb, the more damage it does to the blood supply. His career is over. Under the best of circumstances, we’re trying to save him as a stallion.

“Since he didn’t get to run his race, he’s still full of energy. There are some major hurdles here, and at least a couple of aspects that are very life-threatening. He has to be stabilized. We’re looking at a long surgery that will take hours.”

Dr. Celeste Kunz, the former veterinarian for the New York Racing Association who was instrumental in saving the life of Derby-Preakness winner Charismatic when he broke down in the 1999 Belmont Stakes, provided details of the injury after talking to Dr. Bramlage last night.

“Barbaro has a condylar fracture [near the joint] of the cannon bone, and a comminuted fracture [in pieces] of the pastern,” she told The Post. “The good news is, the pieces [of bone] are large [rather than shattered], the swelling is minimal, it’s not a compound fracture [did not break through the skin] and he’s being an excellent patient. The surgery will take place [today].”

The usual practice for an injury like this is to fuse the bones together with metal plates. New Bolton has an equine pool, which greatly aids recovery by providing support without the horse having to put weight on the repaired leg.

Who knows what might have been if Barbaro did not break down. Seemingly, the Preakness set up perfectly for him. Like Now, breaking from post 1, quickly shot to the front under Garrett Gomez, pressed by Sweetnorthernsaint with Kent Desormeaux up, and Bernardini tucked in behind them saving ground.

Down the backside, Brother Derek ranged up outside into striking position, with Bernardini continuing to stalk the pace. On the turn for home, as Like Now began to back up, Sweetnorthernsaint took the lead, but Bernardini swung off the rail, took command at the top of the stretch and drew clear.

“I didn’t think he’d win by that big a margin,” trainer Tom Albertrani said of his A.P Indy colt, who won a maiden race at Gulfstream Park and the Withers Stakes at Aqueduct in his prior two starts. “With Barbaro in there, I don’t know what margin he would have won by if he didn’t have the injury.”

For Castellano, a 28-year-old native of Venezuela who’s a regular on the New York circuit, it was the biggest victory of his life and first in a Triple Crown event.

“I’m sorry for Michael Matz and the owners [of Barbaro],” he said, “but my horse is a really nice horse. I knew at the three-eighths pole [Bernardini would win]. I had plenty of horse, and the two horses in front of me started tiring. At the quarter-pole, when I asked him he took off.”

Bernardini is a homebred owned by Sheikh Mohammed of Dubai’s Darley Stable. Just the second horse to win the Preakness in the past 23 years without running in the Kentucky Derby, he ran the mile-and-three-sixteenths in 1:54.3 to pay $27.80, topping a $171.60 exacta, $3,912.80 trifecta and $11,151.20 superfecta.